Convert String to Byte Array in PHP

<?php 
    $str = 'nilpointer.net';
    $hex = array();
    foreach (str_split($str) as $chr) {
        $hex[] = sprintf("%02X", ord($chr));
    }
    echo implode(' ', $hex);
?> 

Output

6E 69 6C 70 6F 69 6E 74 65 72 2E 6E 65 74